It has been known since the work by Butlerow and Loew (Ann. 120, 295 (1861) and J. prakt. Chem. 33, 321 (1886) in the previous century that the autocondensation of formaldehyde hydrate (formose synthesis) in the presence of basic compounds such as calcium or lead hydroxide is accompanied by the formation of hydroxy aldehydes and hydroxy ketones. Work on formose synthesis has repeatedly been carried out since then.
In this connection one may refer, for example, to Pfeil, Chem. Berichte 84, 229 (1951); Pfeil and Schroth, Chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss. Carbohydrate Research 24, 29-44 (1972); the formoses obtained from glyceraldehyde and dihydroxy acetone according to Emil Fischer; German Pat. Nos. 822,385; 830,951 and 884,791, U.S. Pat. Nos. 2,121,981; 2,224,910; 2,692,935 and 2,272,378 and British Pat. No. 513,708. These known processes have certain disadvantages such as poor volume/time yields and, colored by-products. New processes have recently been developed by which virtually colorless formoses free from undesirable by-products can be prepared in high yields with the aid of conventional catalysts.
One of these new processes consists of carrying out the condensation of formaldehyde hydrate in the presence of catalysts consisting of soluble or insoluble lead (II) salts or of lead (II) ions attached to high molecular weight carriers and in the presence of a cocatalyst which consists of a mixture of hydroxy aldehydes and hydroxy ketones which may be obtained from the condensation of formaldehyde hydrate and is characterized by the following molar ratios:
compounds with 3 C atoms/compounds with 4 C atoms: 0.5 to 2.0; PA1 compounds with 4 C atoms/compounds with 5 C atoms: 0.2 to 2.0; PA1 compounds with 5 C atoms/compounds with 6 C atoms: 0.5 to 5.0; PA1 Li, Na, Ca, Ba, K, Ag, Be, La, Ce, V, Nb, Ta, Mo, W and up to 1% by weight of the elements Ru, Rh, Pd, Au, Ir, and Pt. PA1 25 parts of a polypropylene oxide (hydroxyl number 74) which has been started on ethylenediamine, PA1 22 parts of the formite from Example 3, PA1 10 parts of trichloroethyl phosphate, PA1 15 parts of monofluorotrichloromethane, PA1 0.5 parts of dimethylbenzylamine, PA1 0.5 parts of a commercial silicone stabilizer (L-5420 of UCC) and PA1 75 parts of a commercial phosgenation product of aniline/formaldehyde condensates (isocyanate content: 29%)
and in which the proportion of components containing from 3 to 6 carbon atoms is at least 75% by weight, preferably more than 85% by weight, based on the total quantity of cocatalyst.
The reaction temperature is generally between 70.degree. and 110.degree. C., preferably between 80.degree. and 100.degree. C., and the pH of the reaction solution is adjusted by controlled addition of an inorganic or organic base, first to 6.0 to 8.0, preferably 6.5 to 7.0 up to a conversion of 10 to 60%, preferably 30 to 50%, and thereafter to 4.0 to 6.0, preferably 5.0 to 6.0. It is surprisingly found that the proportions of products in the mixture of polyols, hydroxy aldehydes and hydroxy ketones can be varied in a reproducible manner by this special control of the pH followed by cooling at different residual formaldehyde contents (0 to 10% by weight, preferably 0.5 to 6% by weight).
When the autocondensation of formaldehyde hydrate has been stopped by cooling and/or by inactivation of the lead catalyst with acids, the catalyst, and optionally also the water contained in the products, is removed. Further details of this procedure may be found in German Offenlegungsschriften Nos. 2,639,084 and 2,732,077.
According to German Offenlegungsschrift No. 2,714,084, highly concentrated colorless formoses may also be produced with high volume/time yields by condensing aqueous formalin solutions and/or paraformaldehyde dispersions in the presence of a soluble or insoluble metal catalyst and in the presence of a co-catalyst which has been prepared by partial oxidation of a dihydric or higher hydric alcohol which contains at least two adjacent hydroxyl groups and has a molecular weight of from 62 to 242 or a mixture of such alcohols. The pH of the reaction solution is controlled by controlled addition of a base so that it is maintained at 6.0 to 9.0 until conversion is 5 to 40% complete and is then adjusted to a value from 4.5 to 8.0 until the condensation reaction is stopped so that in this phase of the reaction it is lower by 1.0 to 2.0 units than in the first reaction phase. The reaction is then stopped by inactivation of the catalyst when the residual formaldehyde content is from 0 to 10% by weight, and the catalyst is removed.
High quality formoses can also be prepared by the condensation of formaldehyde in the presence of a metal catalyst and more than 10% by weight, based on formaldehyde, of one or more dihydric or higher hydric low molecular weight alcohols and/or relatively high molecular weight polyhydroxyl compounds (see German Offenlegungsschrift No. 2,714,104).
According to another process, it is particularly economical to prepare formose directly from formaldehyde-containing synthesis gases, i.e. without first preparing aqueous formalin solutions or paraformaldehyde.
For this purpose, the synthesis gases such as can be obtained from the large scale industrial production of formaldehyde are conducted continuously or intermittently at temperatures of from 10.degree. to 150.degree. C. into an absorption liquid which consists of water, monohydric or polyhydric low molecular weight alcohols and/or relatively high molecular weight polyhydroxyl compounds and/or compounds capable of enediol formation as co-catalyst and/or soluble or insoluble metal compounds as catalyst, optionally bound to a high molecular weight carrier, which absorption liquid is at a pH of from 3 to 10. The formaldehyde is directly condensed in situ in the absorption liquid (optionally also in a reaction tube or cascade of stirrer vessels following the container for the absorption liquid), and autocondensation of the formaldehyde is stopped by cooling and/or by inactivation of the catalyst with acids when the residual formaldehyde content in the reaction mixture is from 0 to 10% by weight. The catalyst is finally removed.
For some purposes, mixtures of hydroxy aldehydes, hydroxy ketones and optionally polyalcohols of the kind obtained by the processes described above or by processes known in the art are required to be converted into mixtures of polyalcohols by reduction of the carbonyl group. Such polyol mixtures obtained by the reduction of formoses will hereinafter be referred to as "formite". It is possible, for example, to reduce formose with sodium borohydride from aqueous solution at room temperature (see R. D. Partridge, A. H. Weiss and D. Todd, Carbohydrate Research 24 (1972), 42); but reduction of formose may also be carried out electrochemically, for example.
Many processes are already known for the catalytic hydrogenation of sugars and of formose. Widely differing quantities and types of catalysts are employed, depending on the process. Thus L. Orthner and E. Gerisch Biochem. Zeitung 259, 30 (1933), for example, describe a process for the catalytic hydrogenation of formose in which a 4% aqueous solution of formose is hydrogenated with 170% by weight, based on the quantity of formose, of Raney nickel by a reaction carried out for 7 to 8 hours at 130.degree. C. under a hydrogen pressure of 120 bar. Such a process is, of course, economically unsatisfactory in every respect. In U.S. Pat. No. 2,269,935, a process has been disclosed in which a solution containing approximately 40% by weight of formose is hydrogenated at an acid pH with 20% by weight of nickel catalyst at a hydrogen pressure of 600 to 620 bar and at 120.degree. C. The disadvantage of this variation of the process lies not only in the high operating pressure but also in the low pH, which results in products which are colored green by nickel ions.
In U.S. Pat. No. 2,224,910 a process has been disclosed for the hydrogenation of formose, in which a 40% formose solution is hydrogenated with 30% by weight of Raney nickel, based on the quantity of formose, at a hydrogen pressure of 140 to 210 bar and pH 7 for 4 hours. This process is also unsatisfactory because of the large amount of catalyst required and the long reaction time.
Other hydrogenation processes have been described in German Pat. Nos. 705,274; 725,842; 830,951; 888,096 and 1,004,157 and in U.S. Pat. Nos. 2,271,083; 2,272,378; 2,276,192; 2,760,983 and 2,775,621. All of these processes, however, have one or more of the following disadvantages: considerable outlay in apparatus and difficulty of handling owing to the high hydrogen pressures; large consumption of catalyst, based on the quantity of hydrogenated product (10 to 200% by weight); discolored products due to long hydrogenation times (1 to 10 hours).
Common to all of the known processes is the use of metal catalysts and in some cases noble metal catalysts. Raney nickel, which is commonly used, develops its full activity only in the alkaline range. However, since formoses have a strong tendency to caramelize and give rise to severely discolored products in an alkaline medium, the processes known in the art are generally carried out at a slightly acid or neutral pH.
It was therefore an object of the present invention to provide a process for the rapid hydrogenation of formose with little capital expenditure and very small quantities of catalyst.